U.S. Pat. Nos. 5,500,083; 5,617,975; and 5,628,873 (the disclosures of which are incorporated by reference herein) disclose assorted methods and devices for storing, treating, and discharging comminuted cellulosic fibrous material prior to treatment in a chemical pulping process. Such devices, typically chip bins, are marketed under the trademark DIAMONDBACK.RTM. by Ahistrom Machinery of Glens Falls, N.Y. The DIAMONDBACK.RTM. chips bins have been a remarkably successful innovation that has received widespread acceptance throughout the pulping industry. The DIAMONDBACK.RTM. bins are characterized by uniform movement and treatment without the need for mechanical agitation or vibration that is characteristic of the prior art bins and of the other offerings in the field.
In addition to the single-convergence DIAMONDBACK.RTM. chip bin technology disclosed in these patents, U.S. Pat. No. 5,617,975 also discloses an alternative geometry employing "chisel-type" convergences. In particular FIGS. 2 and 3 of U.S. Pat. No. 5,617,975 disclose a bin discharging arrangement which includes a substantially cylindrical bin having a discharge transition with gradually converging opposite side walls which converge to a substantially rectangular outlet. The substantially rectangular outlet includes a metering screw that transfers the material to a discharge. The chisel-type convergence device has also proven to be effective for uniformly treating and discharging material from the bin while insuring uniform movement of the material through the bin. Vessels having this geometry are marketed under the trademark CHISELBACK.TM. by Ahistrom Machinery.
Although, the CHISELBACK.TM. bin has also been proven to be effective for treating and handling wood chips, research has shown that certain improvements can be made to the CHISELBACK.TM. bin to enhance its performance. For example, due to the uniform movement of material through the upper cylindrical portion and the lower transition portion, it has been found that non-uniform agitation of the material in the outlet below the bin can affect the movement of the material above. Where the DIAMONDBACK.RTM. bin is typically not as sensitive to such non-uniformities (since the DIAMONDBACK.RTM. bin converges to a generally localized, circular or square-shaped discharge) the CHISELBACK.TM. bin, having a relatively elongated discharge, is more susceptible to non-uniformities. For example, when the metering screw of a CHISELBACK.TM. bin transfers material to one side of the bin the horizontal thrust of the screw produces a build up of material on one side of the bin which hampers the movement of material on that side. In addition, the proximity of a screw or other conveyor to the outlet of the bin transition can also hinder the uniform flow of material out of the discharge and thus affect the movement of the material above. These and other deficiencies of the chisel-type bin disclosed in U.S. Pat. No. 5,617,975 are addressed by the present invention
In accordance with the invention a method and apparatus are provided such that: the chips are substantially continuously and uniformly withdrawn from the "chisel"; the upward or lateral thrust of the discharge device into the chip column above is minimized, if not eliminated entirely; and the discharge of the metering device to the subsequent device/conduit is substantially uniform and continuous.
Substantially continuous and uniform withdrawal from the transition heretofore was not typically required when treating comminuted cellulosic fibrous material, nor was the reason for desiring such a uniform withdrawal recognized in this art. In applications dealing with the uniform column movement in digesters, it has only been recently recognized how critical it is to establish uniform flow and treatment throughout the height of a vessel transferring comminuted cellulosic fibrous material. It has recently been learned that this is particularly important in the lower portions of a vessel since conditions there can affect the movement throughout the height of the vessel. Vessels in which the treatment in the vessel is highly dependent upon this uniform movement, such as the vessels in which steam is introduced to heat and displace the air, or other gases or liquids are introduced to treat the material, are particularly sensitive to the effects of non-uniform chip column movement.
The prior art vessels have been characterized by non-uniform withdrawal, as evidenced by vibrating discharges in the chip bins and rotating agitators in the chip bins, impregnation vessels and digesters. Except for the DIAMONDBACK.RTM. bins, substantially uniform discharge was heretofore unknown.
According to an embodiment of the present invention, the chips are preferably continuously withdrawn so that little or no back-up of chips into the chisel transition occurs and the flow across the outlet of the chisel is continuous and as uniform as possible so that the treatment is as uniform and efficient as possible. For example, this substantially uniformity may be exemplified by a method and apparatus in which the retention time in the vessel does not vary more than +/-5 minutes for any individual volume of chips compared to any other individual volume, preferably +/-4 minutes, or less, for a throughput of at least 30 tons of chips per day (e.g. at least about 50 tons per day).
The minimization of upward thrust, or lateral thrust having an upward component is discussed below. Several ways of perfecting this are disclosed, including a height H of a third transition at least as great as the width of the open bottom of a second transition; an adjustable outlet width; and a "step-in" of the outlet to a width narrower than the transition conduit. These options may be used alone or in combination. For example, the lower transition having a height H may be unnecessary if the step-in or adjustable outlet is sufficient to ensure that little or no upward thrust is imposed on the chip column. However, there are many other ways that this could be perfected, in addition to a multi-star-type metering device and a downward-sloping screw. For example, the screw flights could be re-designed so that they are leaning or canted forward instead of backward (as is conventional). Having the flights leaning forward can impose a load on the chips having a horizontal and downward force component and little or no upward component.
Therefore, in one embodiment of the invention, the mechanism for metering chips from the vessel imposes little or no force on the downflowing chip mass that impedes the movement of material in the chip mass.
In addition, if horizontal or upward forces cannot be eliminated, it is preferred that the force be directed against a generally vertical wall of the transition or a wall that is tapering inward and not against a wall tapering outward. The generally vertical wall or a wall tapering inward is less likely to transfer a thrusting force upward into the chip column A wall that is tapering outward undesirably allows the force to be transferred upward along the taper and into the chip column and thus interferes with the column movement.
The continuous discharge is preferred to prevent the discharge of "slugs" of chips to the downstream conduit or device. There are also various ways of achieving this. One way is to modify a multi-chip meter, or other multi-star-type feeder, design so that the pockets of the feeder are offset, or out of phase, so that as the pockets of one star-type feeder are being filled the pockets of another feeder are emptying. Thus a fairly uniform discharge of the metering device is obtained instead of having a "slug" discharged that would occur if the star-feeders were not off-set, that is, synchronous. In addition, instead of the pockets being oriented parallel to the axis of the shaft, the pockets, or their paddles, may also be angled relative to the axis of rotation. The pockets of one angled rotor may also be off-set, or out of phase, with the pockets of the adjacent rotor.
In addition, in order to improve the uniformity with which chips are discharged from the multi-star feeder device, the star-type feeder may be designed to have shallower pockets, or a larger shaft, and operated at a faster rotational speed. Due to the depth of the pockets of a conventional star-type feeder, for example, a Chip Meter, the speed of rotation of the feeder is relatively slow. This allows sufficient time for the pockets to fill as the pocket rotates passed the inlet of the feeder. As a result, when the conventional feeder is rotated to the outlet, a slug of chips falls out of the slowly moving pocket as the pocket is exposed to the outlet. However, by designing a shallower pocket and rotating the rotor faster (e.g. at least 10% faster than the conventional maximum operating rpm), a more uniform, non-slug-like discharge of chips can be obtained. Furthermore, the pockets of two or more star-type feeder rotors can be mounted out of phase so that a relatively uniform discharge of material is obtained. Again, the pockets may also be oriented so that they are not parallel to the axis of rotation. If necessary, circumferential "mid-feathers", or barriers, may be used to improve the uniformity of the filling of the pockets and prevent "short circuiting" of the material passed the flights of the feeder.
Another similar metering device that can be used to ensure a relatively uniform discharge of chips is one or more screw-type devices that pass the chips from an upper inlet to a lower outlet instead of transferring the material horizontally. Similar to the multi-star feeder discussed above, the screw-type device can be designed with a relatively shallower screw flight height and operated at a higher speed of rotation. For example, the screw according to the invention may have the same profile as the rotors as described above. This metering screw may also extend across the outlets or transitions such that no baffles are necessary. Again, if necessary, circumferential "mid-feathers", or barriers, may be used to improve the uniformity of the filling of the flights and prevent "short circuiting" of the material passed the flights of the screw. Also, to ensure uniform discharge, the screw may contain more than one continuous screw flight, for example, at least two continuous parallel flights may be used.
Also, if the metering device can not itself provide a uniform discharge of material, the device following the metering device can provide the uniform discharge. For example, if the multiple-star-type feeder discussed above comprises or consists of one or more conventional, deep-pocket, slow rotational feeders, these one or more feeders can discharge to a device, for example, a horizontal feed screw, that provides a uniform discharge of material to an outlet.
There are many ways that the discharge device may be designed. However, preferably the present bin for handling comminuted cellulosic fibrous material includes: a mechanism or method for substantially continuously and uniformly withdrawing material from the "chisel"; a mechanism or method for minimizing the upward thrust into the chip column above it; and a mechanism or method for substantially uniformly and continuously discharging the material from the metering device.
One generalized embodiment of the present invention comprises or consists of a "chisel-type" discharge as shown in U.S. Pat. No. 5,617,975 in which the flow of material through the bin is not hindered by the way in which the material is discharged from the bin. For example there may be provided: A bin for handling comminuted cellulosic fibrous material comprising: a hollow substantially right circular cylindrical first, main body portion having a substantially vertical central axis, a top and an open bottom; a top wail closing off the top of the main body portion, and allowing introduction of particulate material into the hollow main body portion mounted thereon; a second hollow transition, portion connected to the bottom of the first body portion having a substantially circular cross-section open top and a substantially rectangular cross-section open bottom and a first width dimension, and a larger cross-sectional area at the top thereof than at the bottom thereof, and opposite non-vertical gradually tapering side walls; at least one metering device mounted below the open bottom of the second transition portion, in a housing; a third, hollow transition, portion located between the second hollow transition portion and the metering device housing and having a height; a discharge operatively connected to the metering device housing; and the at least one metering device being operable to move particulate material from the bottom of the third transition portion to the discharge; and wherein the height of the third hollow transition portion is at least equal to the first width dimension of the open bottom of the second hollow transition portion. In a preferred embodiment of the present invention, the third hollow transition portion has a second width dimension and the second width dimension is greater than the first width dimension.
Preferably one or more baffles is or are provided between the second transition portion and the third transition portion for extending the converging side walls of the second transition portion into the third transition portion, and to provide radial relief from material flowing into the third transition portion. Preferably the one or more baffles is or are adjustable (preferably two adjustable baffles are provided) to adjust the width of the open bottom of the second transition portion to regulate the flow of material from the second transition portion through the third transition portion to the metering device. The baffles may be spaced from each other a different width from one end of the metering device to another, so that the spacing therebetween increases from a point furthest from the discharge to a point adjacent the discharge.
The metering device may comprise a single variable pitch metering screw. Alternatively the metering device may comprise at least one metering screw that is disposed at an angle of at least about two degrees (e.g. about 2-15 degrees), sloping downwardly from the horizontal from a position furthest from the discharge to a position closest to the discharge. Alternatively the metering device may comprise a plurality of star-type metering devices, and baffles may be provided between the star-type metering devices to direct material into the devices. Also steam introduction into the first body portion may be provided as is conventional per se.
According to another aspect of the invention there is provided a method of handling comminuted cellulosic fibrous material utilizing a chisel-type discharge from a comminuted cellulosic fibrous material bin having a first hollow substantially right circular cylindrical body portion, a second hollow transition portion connected to the bottom of the first portion and having a substantially cross-section open top and a substantially rectangular cross-section open bottom and a larger cross-sectional area at the top than at the bottom and opposite non-vertical gradually tapering side walls, and at least one metering device mounted below the open bottom of the second transition portion in a housing. The method comprises the steps of: (a) Feeding comminuted cellulosic fibrous material into the top of the first body portion. (b) Causing the material to flow downwardly through the first portion and into and through the second portion. (c) Causing the material to flow through a third transition portion from the second portion to the metering device so that the flow of material from one side to the other is substantially uniform. And, (d) discharging the material from the bin using the metering device. There also preferably is the further step (e) of adjusting the size of the opening between the second transition portion and the third transition portion to control the flow rate of material. Step (e) may be practiced in part by providing a difference in the width from one end of the metering device to another end of the metering device, the spacing substantially continuously increasing.
The method may also be distinguished by the further step of steaming material in the bin. The method may still further be distinguished by steps (c) and (d) are practiced so that the third transition is not completely full of comminuted cellulosic fibrous material so as to provide compression relief for the material; or, where the second portion has an open bottom with a first width dimension, and wherein the third transition portion has a height, and steps (b) through (d) are practiced so that the height of the third transition portion is at least equal to the first width dimension of the open bottom of the second transition portion.
According to yet another aspect of the present invention a vessel for handling comminuted cellulosic fibrous material is provided comprising the following components: A first hollow top portion. A second hollow transition portion disposed substantially directly below the first portion, the second transition portion having a larger cross-sectional area at a top portion thereof than at an open bottom portion thereof with a first width, and opposite non-vertical gradually tapering side walls. A third hollow transition portion located substantially directly below the second hollow transition portion. At least one adjustable baffle operatively disposed between the second and third transition portions to adjust the effective dimension of the first width. And, a metering device disposed (e.g. substantially directly) below the third transition portion for transporting comminuted cellulosic material from the third transition portion to a discharge. And, the third transition portion, during operation, not being completely full of comminuted cellulosic material so that compression relief is provided. The vessel may also be distinguished by the at least one baffle comprising two adjustable baffles that are spaced from each other a different width from one end of the metering device to another, so that the spacing therebetween increases from a point furthest from the discharge to a point adjacent the discharge.
According to another aspect of the invention there is provided a vessel assembly for handling comminuted cellulosic fibrous material comprising: A substantially hollow chisel-type vessel including a substantially cylindrical main body portion with opposite side walls gradually converging to a substantially rectangular outlet. A discharge device below the outlet. And, means for substantially continuously and uniformly withdrawing material from the outlet with minimal or no lateral or upward thrust on the material above the outlet, and for substantially uniformly and continuously discharging the material from the metering device.
According to another aspect of the invention there is provided a method of treating comminuted cellulosic fibrous material using a vessel having a hollow main body and a chisel-shaped discharge to a substantially rectangular bottom outlet, and a discharge device below the outlet, the method comprising the steps of: (a) Feeding material into the top of the vessel to flow downwardly toward the bottom. (b) Substantially continuously and uniformly withdrawing material from the outlet. And, (c) operating the discharge device so that there is substantially no upward thrust on the material above the outlet as a result of the discharge device and so that the discharge of material from the metering device is substantially uniform and continuous. Steps (a)-(c) may be practiced so that the residence time of a volume of material in the vessel does not differ, in a twenty-four hour period, from the residence time of any other volume by more than about four minutes.
According to another aspect of the invention there is provided a method of treating comminuted cellulosic fibrous material using a vessel having a hollow main body and a chisel-shaped discharge to a substantially rectangular bottom outlet, and a discharge device below the outlet, the method comprising the steps of: (a) Feeding material into the top of the vessel to flow downwardly toward the bottom. And, (b) substantially continuously and uniformly withdrawing material from the outlet and, discharging the material from the discharge device, so that the residence time of a volume of material in the vessel does not differ, in a twenty-four hour period, from the residence time of any other volume by more than five minutes.
It is the primary object of the present invention to provide an improved chisel-type bin or other vessel for handling comminuted cellulosic fibrous material, such as wood chips, a method of handling chips, with maximum uniformity. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and the appended claims.